Radio receiving circuits



April 17, 1934. 0. D. ISRAEL RADIO RECEIVING CIRCUITS Filed Nov. 1, 1928 INVENTOR.

ATTORNEY.

Patented Apr. 17, 1934 UNITED STATES PATENT OFFICE 6 Claims.

My invention relates to the use of a series of coupled resonant circuits as a radio frequency selector.

In my application Serial No. 211,366, filed August 8, 1927, I have described the use of resonant circuits tunable to the various frequencies of usual broadcasting range, together with capacity coupling between them, which is varied for the several frequencies in a regular manner, so as to maintain a varying coupling co-eflicient which results in passing a uniform width band of frequencies at any setting of the resonant circuits.

Generically the invention of my former application consisted in the use of a series of circuits in a selective net work, which operated according to the laws of resonant circuits, and yet were so coupled as to result in passing a uniform band of frequencies at all times. The present invention is generically similar, but differs in that the coupling between the circuits is taken care of automatically rather than mechanically, and in that much greater efliciency is obtained and the electrical values of the coupling elements are very small, thus permitting the use of inexpensive parts.

Specifically, I provide a series of resonant circuits capacitatively tuned, and couple the circuits both capacitatively and inductively, so arranged in value and phase that the algebraic sum of the two varies in an ideal manner to decrease the over-all coupling coefficient as the frequency to which the resonant circuits are tuned, increases.

Where two resonant circuits are coupled together and tuned to the same frequency, the net result is the exclusion of radio frequencies except for a band which includes frequencies which are greater and less than the resonant frequency. If C is the total capacity of each of two resonant circuits and L is the total inductance of each and they are capacitatively coupled by a capacity C1, then the following equations will hold. In the first place the resonant frequency (f) is equal to 12-11 will represent the distance between peaks of the band which actually passes through the coupled circuits at resonance j. K represents the coupling coefficient.

f =f /1K, and f =f /1+K,

and in like manner K is equal to the coupling capacity C1, divided by the sum of the resonant circuit capacity plus the coupling capacity or C1 K C C If C1 is constant K will increase as C decreases.

From the value 1 fi re is derived the value of C, or

which indicates that as C decreases, f increases. From the values of f2 and f1 by subtraction it will be noted that f2-fi, which represents the frequency band, and which is equal to which is independent of the value of C, and will be constant where the circuits are capacity tuned.

But in the inductively coupled circuits and and the difference,

i f rri-m (which represents the width of the band), if it is to remain constant, will require that k be decreased as ,1 increases.

Since f increases as C is decreased, and in' a capacity coupled series of resonant circuits with the capacity coupling constant, It will increase as C decreases, I am able by arranging the phase so that the Is for the capacity coupling, subtracts from the is for the inductive coupling, so that the total It decreases as the frequency is increased, which is the desired relationship. The total is in resonant circuits which are capacity and induction coupled, is the algebraic sum of the two.

I have illustrated in the drawing a circuit using my present capacity and induction coupled arrangement together with untuned amplifiers.

In the drawing:

Figure 1 is a diagram of my circuit showing three resonant circuits with untuned amplifiers at both ends.

Figure 2 is a section of one of the coupling coils.

The diagram of Figure 1 shows the antenna 1 feeding into the untuned amplifier 2 of one stage (it may be more than one). From this amplifier the radio frequencies of the antenna as amplified pass to the resonant, circuits 3, 4 and 5. From the circuit 5 the selected band of frequencies is impressed on the untuned amplifier 6, of one stage (it may be more than one) from which the selected amplified signal is passed to the audio amplifier, so marked, and of which no illustration is called for. I may place all of the untuned amplifiers at either end of the selector, or separate them as shown, or use a different number at one end from that at the other.

The inductances in the resonant circuits are marked L, and the capacities C. Intermediate the circuits the coupling is both inductive and capacitative. This is easily taken care of by using only a few turns of the inductances for coupling, and shifting the position of the coupling turns so as to get the proper value of C1 and then fixing them in this position. Thus the coupling turns are kept at approximately ground potential and shifted closer to or further away from the high potential end of the main inductance, which shifts the capacity but does not change the mutual inductance to an appreciable degree.

In Figure 2 is shown a coupling coil which consists of the two windings 7 and 8 on the core 9, and the small disc 10 respectively the winding 8 containing a few turns of wire. The position of this disc, which is regulated by the location of its mounting arm 11, is adjustable from the high potential end of the structure to the low potential end, and so positioned as to give the required coupling capacity.

In the arrangement which I have selected, using commercial sized tuning condensers for C, in each resonant circuit, requires a value of M for the inductive coupling of 2.5 10- henries, and a capacity coupling of .5 10- farads. These values are very small. All condensers will preferably be alike and be shifted simultaneously by suitable mechanism.

To give a uniform response band of 8 kilocycles between peaks at all the frequencies for which the selector is constructed, the desired value of 7c wfil be or approximately that. Without going into the mathematics, the values given for K1. and K0 in my arrangement, when the latter is subtracted from the former, will give the desired coupling coefiicient with remarkably little variation over the ideal response band, and the width of the band will be very much more uniform than will the response of a tuned radio frequency receiver.

Also the attenuation of the signal is not at all great, as is the case with the usual filter system.

There will be variations of great number in the types of circuits with which my selector will be associated and its manifest advantages in obtaining purity of signal reception, sharpness of tuning and entire freedom from oscillation such as to distort the signal or set up self-generated sounds, will be evident to those skilled in the art.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. In combination in a radio receiver, an untuned radio frequency amplifier element, a nonamplifying signal selecting net work comprising a plurality of tuned circuits, and a second untuned radio frequency amplifier element, in the order named.

2. A signal selective net work, comprising a plurality of resonant non-amplifying circuits, each of said circuits simultaneously tunable to substantially the same desired frequencies, said circuits coupled both inductively and capacitatively, the coupling inductances being fixed, and the capacitative and inductive values adjusted so that the coupling coefficient decreases as frequency increases without a mechanical adjustment of the coupling instrumentalities, in such a way that selectivity remains substantially uniform over the tuning range.

3. A signal selective net work, comprising a plurality of resonant non-amplifying circuits, each of said circuits simultaneously tunable to substantially the same desired frequencies, said circuits coupled both inductively and capacitatively, so that the coupling coefficient decreases as frequency increases without a mechanical adjustment of the coupling instrumentalities, the said inductive coupling being the major coupling, and the said capacitative coupling being out of phase therewith, in such a way that selectivity remains substantially uniform over the tuning range. V

4.. In combination in a radio receiver, an untuned radio frequency amplifier element, a signal selecting net work, and a second untuned radio frequency amplifier element in the order named, said signal selecting net work comprising a plurality of resonant non-amplifying circuits, each of said circuits simultaneously tunable to substantially the same frequencies, said circuits coupled both inductively and capacitatively so that the coupling coefiicient decreases as frequency increases without a mechanical adjustment of the coupling instrumentalities, in such a way that selectivity remains substantially uniform over the tuning range.

5. In combination in a radio receiver, an untuned radio frequency amplifier element, a signal selecting net work, and a second untuned radio frequency amplifier element in the order named, said signal selecting net work comprising a plurality of resonant non-amplifying circuits, each of said circuits simultaneously tunable to substantially the same frequencies, said circuits coupled both inductively and, capacitatively so that the coupling coefiicient decreases as frequency increases without a mechanical adjustment of the coupling instrumentalities, the said inductive coupling being the major coupling, and the said capacitative coupling being out of phase therewith, in such a way that selectivity remains substantially uniform over the tuning range.

6. A signal selective network, comprising a ing out of phase, so that as the tuned frequency increases the summation coupling decreases due to the larger inductive coupling remaining substantially constant while the smaller capacitative coupling out pf phase therewith increases in absolute value.

DORMAN D. ISRAEL. 

